The neutrons are still much lower energy than the 14MeV ones emitted by the D-T reaction.
The idea is hydrogen in the water thermalises the neutrons so that by the time they diffuse through to the B10 they are sufficiently low energy to be captured. The B10+n capture reaction then releases gammas which is what the final few cm of lead is for.
There is also the slight chance that some neutrons will be captured by the hydrogen forming deuterium.
